Static electricity generation has been a recognized problem in the use of electrically insulative materials, such as thermoplastics, in the electronics and medical fields. The phenomenon is the result of the elctrical charge occuring due to rubbing or separating generally insulative materials, resulting in the transfer of electrons across the surfaces. When this happens an electrical charge is isolated within the area of generation unless the electrical conductivity of the material is sufficient to dissipate the charge to the surrounding materials or environment.
The two basic systems presently used for electro-static discharge control (ESD) for plastics are: (1) decreasing volume resistivity of the plastic to a value of less than 10 to the 10th ohm centimeters, and (2) decreasing the surface resistivity to less than 10 to the tenth ohms/square. However, since the only available mechanism for dissipating a charge from a part must occur at a surface, decreasing volume resistivity proportionately decreases surface resistivity, controlling ESD.
Reduction of volume resistivity of thermoplastics is presently accomplished by the addition of various conducting materials to the formulation of the plastic resin. Some plastics manufacturers provide plastics that are filled with metallic powder, flakes or fibers, such as copper, stainless steel, aluminum or carbon. This makes the plastic permanently conductive, but has certain disadvantages because of the mechanical properties of the fillers and the relatively high percentage required to achieve sufficient conductivity to control ESD. Such highly loaded plastics have reduced moldability and formability, degraded surface textures, greater susceptibility to water or other liquid absorption, modified or limited coloration (black only in the case of the most commonly used carbon powder fill), and exhibit a tendency to spall off contaminant particles from the surface in a characteristic known as the "crayon effect".
Another method used for controlling ESD by reducing volume resistivity is the addition of hygroscopic surfactants to the formulation of the plastic resin. The presence of the surfactant at the surface will absorb humidity from the air and create an ionic film that is conductive in reasonably normal relative humidity conditions, and is perceptibly greasy or soapy to the touch. The conductive layer is then a thin film of a water solution of surfactant. If it is dried out under heat or very low relative humidity the conductivity may be significantly reduced, approaching the level of the unfilled plastic. Washing or rinsing the surface with nearly any liquid will remove the surface film instantaneously, returning the resistivity to a level comparable to the unfilled plastic and therefore losing all ESD control.
After a period of time more of the surfactant filler will migrate to the surface to re-establish conductivity. Aging and washing or rinsing removes the antistatic properties of these plastics. Analyses were performed of age-failed anti-static materials of this type that no longer had a conductive surface. It was been found that only the surface was non-conductive, and there was an abundance of conductive material just a 0.003 to 0.005 inches below the surface. This indicates clearly that the surfactants used are only capable of migrating a few thousandths of an inch, and the conductivity of the volume of the core of plastic is not significant. The migration of the surfactant is limited to a very thin skin representing the chemical migration distance of the surfactant molecules through the plastic. The disadvantages of ESD control using surfactant fillers are not merely in the poor-to-uncertain aging and cleaning characteristics, but also in significant losses of mechanical properties of the plastic, affecting both the manufacturability and service life of manufactured parts.
The second basic method for ESD control is the application of a topical coating of surfactants to the surface of an otherwise non-conductive plastic part. Since thermoplastics are slightly hygroscopic, the coating (usually a water or alcohol solution of a surfactant) adheres to the plastic in at least a monomolecular layer. Such topical coatings are usually wiped or sprayed onto the plastic. Unfortunately the durability of the coating is approximately proportionate to the thickness of the application. An excess is usually applied, leaving a film of oily residue on the surface that represents a significant contamination problem in clean environments. Such topical coatings are easily removed by heat, age, washing, rinsing and abrasion, and have proven to have a very limited effective life.
Unfortunately, this process is widely used and the plastic parts are often embossed "ANTI-STATIC". Since the embossing is permanent and the topical treatment is not, it is common for sensitive components to be damaged by ESD in a container that is clearly identified as anti-static that has lost its conductivity.
The purpose of the present invention is to provide a method for producing anti-static thermoplastic articles that have long service life without sacrificing the color, appearance, formability and mechanical properties. It is a further purpose of the invention to provide a method for producing thermoplastic articles that have an ESD controlling armor in the form of a penetrating coat that has a thickness at least equal to the chemical migration distance of surfactant volume filled plastics. It is another purpose of this invention to provide a more permanent anti-static surface armor for plastics in which the plastic surface is modified by the inclusion of a hygroscopic metallic salt compound and an additional dry-conductive metallic salt compound.